This invention relates to the manufacture of very small dimensioned devices such as integrated circuits and, more particularly, to high resolution pattern delineation by plasma-assisted etching in the manufacture of such devices.
In the manufacture of integrated circuits and other very small dimensioned devices, lithographic processes are used to transfer a layout by a designer to a corresponding high resolution pattern on a surface above an appropriate substrate. Such processes normally consist of a masking operation in which the pattern is first defined in a masking layer above the surface to be patterned and a subsequent pattern transfer operation in which the pattern in the mask layer is transferred to the inderlying surface. Formerly, the pattern transfer process included wet chemical etching which provides isotropic etching of the surface, i.e., the etch rate is essentially the same in all directions. Currently, so-called very-large-scale-integration (VLSI) devices, such as the 64 kilobit dynamic random access memory (RAM), having device feature dimensions of less than 2 .mu.m are being developed. The manufacture of such devices requires the use of pattern transfer techniques capable of delineating very fine features. For such fine feature delineation, wet etching techniques have the disadvantage in that the isotropic nature of such etching causes undercutting beneath the masking layer and, consequently, results in poor feature size control.
Recently, there have been developed various plasma-assisted etching techniques which under proper conditions can provide anisotropic etching of a surface, i.e., the etch rate is higher in one direction (usually normal to the surface being etched) than in other directions. Since an anisotropic etch causes less undercutting, it is better suited to high resolution pattern transfer than an isotropic etch. In general, plasma-assisted etching techniques involve the exposure of the surface to be etched to a plasma contained in an apparatus. The plasma is normally generated by the application of an RF electric field across an appropriate gaseous ambient between a pair of electrodes in the apparatus.
One of the plasma-assisted etching techniques called reactive sputter etching is known to provide a particularly high degree of anisotropy, i.e., etching takes place substantially only normal to the surface being etched. Reactive sputter etching is typically performed with the gaseous ambient at a relatively low pressure (e.g., 1.times.10.sup.-3 to 0.1 Torr) and the device to be etched supported by the electrode which is driven by the source of RF power. Etching takes place through sputtering of the surface being etched with chemically reactive ion species. Since under the low conditions in the plasma the ions impinge on the surface being etched largely at normal incidences and a chemical reaction occurs between the impinging ions and the material of the surface, etching takes place anisotropically and rapidly.
In addition to anisotropy, it is also important for many applications for the etching process to be highly selective. That is, the etch rate of the material being etched must be much higher than that of the masking layer and that of the next underlying layer.
There has been considerable effort recently directed towards developing etching processes for high resolution pattern transfer to surfaces of silicon materials, particularly for the patterning of polycrystalline silicon (polysilicon) layers. The term "silicon materials" refers to materials of predominantly elemental silicon composition. Such materials may be single crystalline, polycrystalline, or amorphous and may be doped or undoped. In the manufacture of VLSI silicon-gate metal-oxide-semiconductor (MOS) devices, such as the 64 kilobit dynamic RAM, polysilicon layers are patterned to form capacitor plates of memory cells, gate electrodes of transistors and circuit interconnections. In such devices, the etching process for patterning the polysilicon layer must be both highly anisotropic to provide accurate feature size control and highly selective with respect to silicon dioxide to avoid removal to a very thin (as thin as 500 Angstroms) layer of silicon dioxide which underlies the polysilicon layer in some regions of the device.
Reactive sputter etching of doped or undoped silicon materials in a pure chlorine (Cl.sub.2) plasma is known to provide a high degree of anisotropy, a high etch selectivity with respect to silicon-dioxide and a rapid etch rate. The characteristics of such etching are described, for example, in "Reactive Ion Etching of Silicon" by G. C. Schwartz and P. M. Schaible, Journal of Vacuum Science and Technology 16(2) March/April 1979, p. 410 and in "Anisotropic Polysilicon Etching with Cl.sub.2 Plasma" by W. W. Yao and R. H. Bruce, Electrochemical Society Extended Abstracts, Volume 81-2, October 1981, p. 652. The silicon to silicon-dioxide etch selectivity in reactive sputter etching of silicon materials in a chlorine plasma is enhanced (up to 25:1) if the driven electrode of the etching apparatus is covered with a layer of alumina (Al.sub.2 O.sub.3). The use of alumina as a protective cover of the driven electrode in plasma-assisted etching apparatus is disclosed and claimed in copending and commonly assigned U.S. patent application Ser. No. 295,531, filed on Aug. 19, 1981.
However, there is a problem in using reactive sputter etching of silicon materials in a chlorine plasma in that such an etching process cannot easily be carried out in an apparatus that has been used in another etching process involving a plasma generated in a fluorine-containing compound such as nitrogen trifluoride or trifluoromethane. For example, in the manufacture of the aforementioned 64 kilobit dynamic RAM, the etching of one of the polysilicon layers used in that device is carried out in two stages. An anisotropic etching stage is first used to define a high resolution pattern in that layer followed by an isotropic etching stage to remove undesired polysilicon filaments left by the anisotropic etch. Owing to the advantages discussed above, it is desirable to use chlorine plasma reactive sputter etching for the anisotropic etching stage. For the isotropic etching stage it is advantageous to use reactive sputter etching in a nitrogen tirfluoride (NF.sub.3) plasma which provides a very rapid etch rate and very high etch selectivity (up to 40:1) with respect to silicon dioxide. Plasma-assisted etching of silicon materials in a nitrogen trifluoride plasma is disclosed and claimed in commonly assigned U.S. Pat. No. 4,310,380. It is also desirable in a manufacturing operation to carry out both stages of polysilicon etching in the same etching apparatus in order to achieve a high throughput for the apparatus. However, exposure of the apparatus to the nitrogen trifluoride plasma during the isotropic etching stage causes the apparatus to be passivated for chlorine reactive sputter etching of silicon materials. Such passivation causes chlorine reactive sputter etching of silicon materials to proceed very slowly, if at all, in the apparatus. This passivation problem, which is related to the "black silicon" problem reported in the above-cited reference by G. C. Schwartz et al., has been found to depend on the presence of an inorganic oxide material (e.g., SiO.sub.2 or Al.sub.2 O.sub.3) on the driven electrode of the apparatus and the exposure of such a material to a plasma generated in a fluorine-containing compound (e.g., NF.sub.3, CHF.sub.3 or C.sub.2 F.sub.6). The passivation is worsened when the driven electrode is covered with alumina.
Formerly, a passivated apparatus is reactivated by generating a chlorine plasma at a high RF power for a long period (typically 30 minutes) or by washing the apparatus in a 1 M solution of hydrochloric acid. However, neither reactivation procedure is practical in a manufacturing etching operation owing to the long delays involved. Therefore, a need exists for a method which is feasible in a manufacturing operation for carrying out reactive sputter etching of a silicon material in a chlorine plasma with an etching apparatus that has been exposed to a plasma generated in a fluorine-containing compound. Such a method would significantly improve the manufacture of VLSI silicon-gate MOS devices and other devices requiring high resolution pattern transfer onto the surface of a silicon material.